1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to those suited for electrophotographic printers employing a semiconductor laser as light source.
2. Description of the Prior Art
It suffices for a photosensitive member to be spectrally sensitized to light wavelength of about 650 nm or shorter when the light source used for operating is a gas laser such as a helium-cadmium laser (emission wavelength 441.6 nm) or a helium-neon laser (emission wavelength 632.8 nm). As examples of such a photosensitive member, there are known (1) those using a charge transfer complex of polyvinylcarbazole with trinitrofluorenone for a photosensitive layer, (2) those employing a selenium-sensitized tellurium vacuum deposition layer as a photosensitive member, (3) those comprising a conductive layer, a charge transport layer of selenium vacuum-deposited on the conductive layer, and a selenium-tellurium layer vacuum-deposited on the charge transport layer, (4) those using cadmium sulfide spectrally sensitized with a pigment sensitizer, for a photosensitive layer, and (5) those comprising two separately functioning photosensitive layers: a charge generation layer, which contains an organic pigment to extend the sensible wavelength range to the longer side, and a charge transport layer.
Meanwhile, semiconductor laser transmitters of small size and of low cost have been developed in recent years which additionally can be directly modulated. However, these semiconductor lasers in many cases have emission wavelengths of 750 nm or longer and the above-mentioned photosensitive members are insensitive at all or almost completely to the light of 750 nm or longer in wavelength, so that it is difficult to use semiconductor lasers for electrophotographic printers.
Although attempts have been made to extend the maximum sensible wavelength of photosensitive members to 750 nm or longer by sensitization, these have the following disadvantages: The layer structure of photosensitive layer becomes complicated and setting of operational conditions in the production of photosensitive members becomes more difficult; sensitizing pigments or dyes used fade out during repetitions of light exposure and electric charging, and eventually image recording with semiconductor lasers becomes infeasible; and said photosensitive members comprising separately functioning charge generation and charge transport layers, although sensitized to extend their sensible wavelength range to the longer side, cannot obtain adequate sensitivity and exhibit undesirable photomemory, thereby causing various troubles.